You need to localized the framework resource bundles.
The alert strings are in controls.properties:
Program Files\Adobe\Flex Builder 3\sdks\3.2.0\frameworks\projects\framework\bundles\en_US\src\controls.properties
My Flex 3 Cookbook post tells you how to create localized framework files for a new language:
If this post answered your question or helped, please mark it as such.
We overide the Alert.okLabel by our own text since okLabel is static variable before loading the message resources.
Alert.okLabel = getText("okLabel");
Alert.yesLabel = getText("yesLabel");
Alert.noLabel = getText("noLabel");
Alert.cancelLabel = getText("cancelLabel");